With the recent wide spread use of portable electronic devices, such as mobile phones and notebook personal computers, there has been a strong demand to develop small, lightweight nonaqueous electrolyte secondary batteries having high energy density. There has been also a demand to develop high-output nonaqueous electrolyte secondary batteries serving as batteries for electric vehicles, including hybrid vehicles. Among nonaqueous electrolyte secondary batteries that satisfy these demands are lithium-ion secondary batteries. A lithium-ion secondary battery includes a negative electrode, a positive electrode, an electrolyte solution, and the like and uses materials capable of releasing and storing lithium as negative and positive electrode active materials.
At present, lithium-ion secondary batteries are actively being researched and developed. Among others, lithium-ion secondary batteries using a multilayer or spinel lithium-metal composite oxide as a positive electrode active material output a 4V-class high voltage and therefore are being commercialized as batteries having high energy density. Among main lithium-metal composite oxides that have been proposed are lithium-cobalt composite oxides (e.g., LiCoO2), which are synthesized relatively easily, lithium-nickel composite oxides (e.g., LiNiO2), which use nickel, which is cheaper than cobalt, lithium-nickel-cobalt-manganese composite oxides (e.g., LiNi1/3Co1/3Mn1/3O2), and lithium-manganese composite oxides (e.g., LiMn2O4).
To obtain excellent initial capacity characteristics or cycle characteristics, there have been developed many batteries using a lithium-cobalt composite oxide as a positive electrode active material, and various fruits have already been produced. However, a lithium-cobalt composite oxide uses a cobalt compound, which is expensive, as a raw-material. Accordingly, the cost per unit capacity of a battery using a lithium-cobalt composite oxide is much higher than that of a nickel-hydrogen battery and significantly restricts the applicability thereof as a positive electrode active material. For this reason, there is a high expectation that the cost of the positive electrode active material will be reduced with respect to not only small secondary batteries for use in portable devices but also large secondary batteries for power storage purposes or for use in electric vehicles or the like and thus cheaper lithium-ion secondary batteries will be produced. The fulfillment of such an expectation can be said to be industrially significant.
A lithium-nickel composite oxide using nickel, which is cheaper than cobalt, exhibits a lower electrochemical potential than a lithium-cobalt composite oxide. For this reason, lithium-ion secondary batteries using a lithium-nickel composite oxide are expected to have higher capacities. Also, such lithium-ion secondary batteries exhibit high voltages, as with cobalt-based lithium-ion secondary batteries, and therefore are actively being developed. However, if a lithium-ion secondary battery is produced using a lithium-nickel composite oxide synthesized from lithium and only nickel as a positive electrode active material, it disadvantageously exhibits poor cycle characteristics and is more likely to impair battery performance due to the use or storage in a high-temperature environment compared to a cobalt-based lithium-ion secondary battery. As a lithium-nickel composite oxide to overcome these disadvantages, there are commonly known lithium-nickel composite oxides where nickel is partially substituted by cobalt or aluminum, for example, as disclosed in Patent Literature 1.
As a common method to produce a lithium-nickel composite oxide serving as a positive electrode active material, there is known a lithium-nickel composite oxide production method involving preparing a nickel composite hydroxide serving as a precursor by neutralization-crystallization, mixing the precursor and a lithium compound such as lithium hydroxide, and firing the mixture. However, a lithium-nickel composite oxide synthesized using this method still contains unreacted lithium hydroxide. The unreacted lithium hydroxide may gel a positive electrode mixture material paste obtained by kneading the positive electrode active material. Also, if such a positive electrode active material is charged in a high-temperature environment, the reacted lithium hydroxide may be oxidatively decomposed into gas.
In view of the foregoing, Patent Literature 2 proposes a method of removing lithium hydroxide from a synthesized lithium-nickel composite oxide by adding natural water to the lithium-nickel composite oxide and stirring the mixture. Patent Literature 3 proposes a method of removing an unreacted alkali component from a fired lithium-nickel composite oxide by cleaning the lithium-nickel composite oxide with water.
Patent Literature 4 proposes a method involving adding natural water to a synthesized lithium-nickel composite oxide and stirring the mixture to remove lithium hydroxide and then heat-treating the resulting composite oxide in an oxygen atmosphere having an oxygen concentration of 80% by volume at a temperature of 120° C. or more and 550° C. or less. A positive electrode active material obtained using this method compensates for lithium in the surface of the particles thereof lost when water-cleaned, with lithium inside the particles. Thus, the positive electrode active material has no lithium loss in the particle surface thereof, allowing for reductions in the positive electrode resistances of batteries.